1. Field of the Invention
The invention relates to an optical apparatus, and relates to an optical apparatus for use in an optical information recording/reproducing apparatus for writing and/or reading a signal to/from an optical disc used as an optical information recording medium.
2. Description of Related Art
An optical disc has an information recording surface and a transparent substrate to protect the information recording surface. Information is written or read out to or from the information recording surface of an optical disc by irradiating a laser beam onto the information recording surface through the transparent substrate.
As for the optical disc, the improvement in its recording density is being pursued. In order to write or read out information to/from the information recording surface of an optical disc having improved recording density, it is necessary to converge the laser beam to a sufficiently small beam on the information recording surface. As a method to converge the laser beam into a small beam, it is attempted to raise the numerical aperture of the objective lens. Hitherto, the numerical aperture of the objective lens has generally been equal to or about 0.45. It is now contemplated to raise the value to a range between 0.6 and 0.85.
Since the laser beam has been designed so as not to cause an aberration for a transparent substrate having a predetermined thickness, a problem was that a spherical aberration occurs when the transparent substrate has a thickness error. Particularly, when using an objective lens with a large numerical aperture, as compared with a case of using an objective lens with a small numerical aperture, it is easily influenced by the thickness error of the transparent substrate and a large spherical aberration occurs even with a small thickness error.
When reproducing an optical disc having a multilayer structure, since a thickness of light transmitting substrate differs in dependence on the layer, a problem was that the spherical aberration occurs so that it is made difficult to reproduce the signal in a preferable way. On the other hand, a method has been deviced in which a liquid crystal device divided concentrically is used, a refractive index of the concentric portion is changed by applying a voltage to the liquid crystal devices, so as to effect the correction of a spherical aberration of the optical disc having multilayer structure. Such a method is disclosed in Japanese Patent Kokai No. 10-269611.
The liquid crystal device, however, has a problem such that its response speed is low and the device cannot respond to a spherical aberration which fluctuates in one circumference of the substrate thickness of the optical disc. The problem of the occurrence of the spherical aberration occurs typically in the case where a numerical aperture of an objective lens is increased. In this case, preferable recording and reproduction cannot be performed unless thickness precision of the optical disc is set severely.